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Perhaps the most unavoidable climate story of 2012 was the
warmth that gripped much of the United States, and to a
lesser degree, the planet, throughout the entire year. Heat waves
brought "spring in March" to parts of the country, and broke
all-time high-temperature records in a number of places. This,
inevitably, led to a discussion of global warming and the degree
to which it contributes to some types of extreme weather, in this
case heat waves.

In fact, prominent climate scientist James Hansen, of NASA's
Goddard Institute for Space Studies, and colleagues published
research saying recent heat waves "were
a consequence of global warming, because their likelihood in
the absence of global warming was exceedingly small." Some other
climate scientists, however, disagreed about the degree to which
heat waves can be attributed to climate change.

Meanwhile, many of the top climate stories this year have become
something like annual rites recently, as people around the world
grapple with human-caused climate change, and attempt to address
it and its effects. [ 7
Hottest Climate Change Stories of 2012 ]

Natural disasters, such as
Hurricane Sandy (actually a hybrid storm) this year
like others last year, have sparked discussion of the
connection between climate change and increased risk for some
extreme weather events. A majority of Americans also seem to be
making the connection between
extreme weather and climate change, according to surveys by
the Yale Project on Climate Change Communication and
the George Mason University Center for Climate Change
Communication.

In reality, attributing any single weather event to global
warming is tricky, though some scientists said the planet's
increasing temperatures may have worsened Sandy. "The climate
influences on this are what we might call the 'new normal,' the
changed environment this storm is operating in," Kevin Trenberth,
who heads the climate analysis section of the National Center for
Atmospheric Research, told LiveScience at the end of October. For
instance, the warmer ocean surfaces — which fuel hurricanes — may
increase the risk that a storm will become more intense,
Trenberth said. In addition, rising sea levels worsen the risk of
flooding, the cause of much of the devastation Sandy wrought.

Likewise, global climate talks moved forward slowly, as they have
in the last few years, against warnings that nations must curb
the planet's rising greenhouse gas emissions or face dramatic
consequences.

This year also brought some milestones.
Arctic sea-ice cover retreated to a record low in September.
As with unusually warming temperatures, the record sea-ice
retreat did not come out of the blue. In recent years, the
sea-ice cover has fallen below the average extent for 1979 to
2000, and, likewise, the first decade of this century was the
warmest decade ever recorded in all continents of the globe,
according to the World Meteorological Organization.

Scientists who study sea ice have blamed a combination of natural
fluctuations and human-caused warming for the increased loss of
ice, although some differ as to how much humans have contributed,
Claire Parkinson, a senior scientist who studies climate at
NASA's Goddard Space Flight Center, said in September.

Early in the year, the United States, once the biggest
contributor to greenhouse gas emissions, saw its carbon-dioxide
emissions from energy use drop to the lowest level since 1992, a
decline the Department of Energy attributed to a mild winter, a
shift from coal to natural gas and a slow economy. In 2011, the
United States contributed 16 percent to the world's emissions
from fossil fuel use, behind the 28 percent contribution from the
top emitter, now China, according to
a report by the Global Carbon Project.